What times itself three times is 81?

A number multiplied by itself 3 times is denoted by the number, say, x, to the power of 3, or x cubed:

x³

a shorthand way of saying

x · x · x

Since you want to know which number cubed is equal to 81, set this equal to 81.

x³ = 81

Take the cube root of each side (or calculate 81 to the power).

x ≈ 4.3267

x ≈ 4.3267